Past approaches for antenna design include spirals that are not sufficiently compact since their absorber cavities have generally been on the magnitude of a quarter wavelength deep. For example, an antenna with frequency of 10 GHz which has a wavelength of approximately one inch requires a cavity of at least a quarter inch in depth. Since this past approach matches the cavity's depth to that of the longest wavelength, it is not suitable for broadband operations.
Other past approaches f or compact antennas include utilizing patch antennas. Patch antennas are relatively thin and can be on the order of 2% of lambda (i.e., wavelength) in thickness. However, patch antennas are limited in bandwidth and are too large for certain applications where space is considered a premium. Moreover, patch antennas cannot be dedicated to multioctave bandwidths.
Still another previous approach is the multioctave bandwidth spiral-mode microstrip (SMM) antenna. However, this approach necessitates the use of a large ground plane that extends past the diameter of the spiral arms of the antenna in order to operate. This large ground plane increases the overall size of the antenna which may not be suitable for applications that demand a relatively small antenna. Moreover, the SMM antenna approach can only provide a single common ground plane for a dual or multiple concentric antenna configuration. This greatly limits isolation between the antennas.
Accordingly, there is a need for a compact spiral antenna that has multioctave bandwidth capability that allows isolation between concentric spirals.